The demand for rechargeable batteries having ever greater energy density has resulted in substantial research and development activity in rechargeable lithium batteries. The use of lithium is associated with high energy density, high battery voltage, long shelf life, but also with safety problems (ie. fires). As a result of these safety problems, many rechargeable lithium battery electrochemistries and/or sizes are unsuitable for use by the public. In general, batteries with electrochemistries employing pure lithium metal or lithium alloy anodes are only available to the public in very small sizes (eg. coin cell size) or are primary types (eg. non-rechargeable). However, larger rechargeable batteries having such electrochemistries can serve for military or certain remote power applications where safety concerns are of somewhat lesser importance.
Recently, a type of rechargeable lithium battery known as lithium-ion or `rocking chair` has become available commercially and represents a preferred rechargeable power source for many consumer electronics applications. These batteries have the greatest energy density (Wh/L) of presently available conventional rechargeable systems (ie. NiCd, NiMH, or lead acid batteries) Additionally, lithium ion batteries operate around 31/2 volts which is often sufficiently high such that a single cell can suffice for many electronics applications.
Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. Insertion compounds are those that act as a host solid for the reversible insertion of guest atoms (in this case, lithium atoms). The excellent reversibility of this insertion makes such compounds function extremely well in rechargeable battery applications wherein thousands of battery cycles can be obtained. In a lithium ion battery, lithium is extracted from the anode material while lithium is concurrently inserted into the cathode on discharge of the battery. The reverse processes occur on recharge of the battery. Lithium atoms travel or "rock" from one electrode to the other as ions dissolved in a non-aqueous electrolyte with the associated electrons travelling in the circuit external to the battery.
3.6 V lithium ion batteries based on LiCoO.sub.2 /pregraphitic carbon electrochemistry are now commercially available (eg. products of Sony Energy Tec. or A&T Battery). Many other lithium transition metal oxide compounds are suitable for use as cathode material, including LiniO.sub.2 (described in U.S. Pat. No. 4,302,518) and LiMn.sub.2 O.sub.4 (described in U.S. Pat. No. 4,507,371). Also, a wide range of carbonaceous compounds is suitable for use as the anode material, including coke (described in U.S. Pat. No. 4,725,422) and pure graphite (described in U.S. Pat. No. 4,423,125). The aforementioned products employ non-aqueous electrolytes comprising LiBF.sub.4 or LiPF.sub.6 salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.
Lithium ion batteries can be sensitive to certain types of abuse, particularly overcharge abuse wherein the normal operating voltage is exceeded during recharge. During overcharge, excessive lithium is extracted from the cathode with a corresponding excessive insertion or even plating of lithium at the anode. This can make both electrodes less stable thermally. Overcharging also results in heating of the battery since much of the input energy is dissipated rather than stored. The decrease in thermal stability combined with battery heating can lead to thermal runaway and fire on overcharge. Many manufacturers have decided to incorporate additional safety devices as a greater level of protection against overcharge abuse. For instance, the aforementioned products of Sony incorporate an internal disconnect device which activates during overcharge abuse (as described in U.S. Pat. No. 4,943,497). Such devices can be effective, but their incorporation introduces additional cost and reliability problems pertaining to the device and its operation. More importantly, the contents of the battery are left in a less thermally stable state, thereby making them more sensitive to mechanical abuse. Internally disconnected batteries cannot be externally discharged to drain them of energy for purposes of safer disposal.
In European Patent Application No. 614,239, Tadiran disclose a method for protecting non-aqueous rechargeable lithium batteries against both overcharge and overtemperature abuse via use of a polymerizing electrolyte. The liquid electrolyte polymerizes at battery voltages greater than the maximum operating voltage or maximum operating temperature of the battery thereby increasing the internal resistance of the battery and protecting the battery. The method is suitable for lithium batteries employing pure lithium metal, lithium alloy, and/or lithium insertion compound anodes. Therein, many requirements are imposed on the bulk electrolyte characteristics simultaneously. The bulk electrolyte must provide acceptable battery performance and yet also serve to provide protection against overcharge and overtemperature abuse. It can be very difficult to identify bulk electrolytes that adequately meet all these requirements and even more difficult to obtain close to optimal results in each case. For example, polymerization may not occur at both the preferred voltage and the preferred temperature. Thus, the operating range of the battery may be restricted undesirably or the preferred safety characteristics may not be obtained. In turn therefore, it may be preferred to provide overcharge and overtemperature protection via independent means.
In the art, it is common to protect against over-temperatures that originate from some types of electrical abuse (eg. short circuit) by using a suitable separator that melts or shuts down at a specific temperature (the shutdown temperature). During short circuit abuse, the internal resistance of the battery increases markedly when the separator melts, thereby protecting the battery. Microporous polyolefin separators are suitable for this purpose. Microporous polypropylene and polyethylene separators, having shutdown temperatures about 155.degree. C. and 125.degree. C. respectively, are commonly employed in lithium batteries.
The preferred embodiment in the aforementioned European Application employs an electrolyte comprising 1,3 dioxolane solvent and LiAsF.sub.6 salt in a battery normally charged to 3.4 volts. The preferred embodiment further required a polymerization inhibitor to prevent undesirable polymerization during normal operation. Suitable electrolytes for use in higher voltage batteries (such as typical commercial lithium ion batteries) were not identified.
It is known in the art that certain aromatic compounds, including heterocyclic compounds, can be polymerized electrochemically (eg. R. J. Waltman et al. investigated the properties of electropolymerized polythiophene in J. Electrochem. Soc., 131 (6), 1452-6, 1984.) Additionally, polymers of certain heterocyclic compounds have been used as electrodes in the development of various electrochemical devices including lithium batteries (eg. as in Japanese Patent Application Laid-open No. 04-272659 of Matsushita).
Some aromatic compounds have been used in electrolyte solvent mixtures and/or as electrolyte solvent additives in certain specific rechargeable non-aqueous lithium batteries. For instance, toluene has been used as an electrolyte solvent and/or electrolyte additive to enhance cycle life (as in Japanese Patent Application Laid-open No. 04-249870) and/or provide a means for activating an internal disconnect device (similar to that described in U.S. Pat. No. 4,943,497 above) on overcharge (as in Japanese Patent Application Laid-open No. 04-332479). No mention is made in any of these applications about potential safety advantages resulting from the electrochemical polymerization capability of the additives. It is unclear whether the actual embodiments in these applications would possess a safety advantage in practice during overcharge abuse as a result of a polymerization of the additives (ie. other events that occur during overcharge might prevent polymerization, such as an earlier activation of the internal disconnect device in the latter Japanese Application, and/or polymerization might not result in an observed safety improvement).
Additionally, some aromatic heterocyclic compounds have been used as electrolyte solvent additives for purposes of enhancing cycle life in certain specific rechargeable non-aqueous lithium batteries. In Japanese Patent Application Laid-open No. 61-230276, a laboratory test cell employing an electrolyte comprising a furan solvent additive demonstrated an improved cycling efficiency for plated lithium metal. In Japanese Patent Application Laid-open No. 61-147475, a polyacetylene anode, TiS.sub.2 cathode battery employing an electrolyte comprising a thiophene solvent additive showed better cycling characteristics than similar batteries without the additive. No mention is made in either of these applications about potential safety advantages resulting from the electrochemical polymerization capability of the additives. Also, it is unclear whether the actual embodiments in these applications would possess a safety advantage in practice during overcharge abuse as a result of incorporating the additives (ie. other events that occur during overcharge might prevent polymerization and/or polymerization might not result in an observed safety improvement).